1. Field of the Invention
This invention relates to a method for coating a substrate, more particularly to a method for coating metal cookware with a very hard and non-stick coating.
2. Description of Related Art
U.S. Pat. No. 4,862,609 discloses an ironing sole plate that has a composite coating of a mechanically resistant compound. The sole plate is coated by applying a layer of mechanically resistant ceramic or metallic material to the sole plate's aluminum base body and subsequently applying an organic bonding agent, composed of a resinous binding material enriched by PFE, PFA or silicone, to the mechanically resistant layer. It is noted that the application of an excessively thick coating of the bonding agent to the mechanically resistant layer, although it increases the non-stick ability of the sole plate, will reduce its scratch resistance. Therefore, only a thin coating of the bonding agent is applied, having a mean thickness of below ten micrometers, preferably between 0.01 and 2 micrometers. In addition, other coating methods have been disclosed in U.S. Pat. Nos. 3,900,684, 4,070,525, 4,196,256, 4,252,859, 4,395,445, 4,568,573, 4,683,168 and 4,808,490. When the previous techniques are combined and are adopted to manufacture coated metal cookware, the resulting product oftentimes has a hard cooking surface but greasy dirt is liable to accumulate on the cooking surface since a thin coating consisting of PTFE cannot provide sufficient non-stick ability. Since the greasy dirt is difficult to remove and causes a reduction of a pan's heat conductivity efficiency; cookware thus afflicted often must be subjected to excessive heat in order to obtain desired results. When an excessively thick coating consisting of PTFE is applied in order to increase the non-stick ability and prevent the accumulation of greasy dirt, the hardness of the surface is decreased, thereby reducing the scratch resistance.